Although active noise cancellation systems have been developed, particularly for use in building ventilation ducts, previously known systems are not well adapted for use in the environment of motor vehicles. A large of number of patents are directed to improvements in the electronics and signal processing techniques for generation of the noise cancellation signal. For example, U.S. Pat. No. 4,473,906 to Warnaka et al., U.S. Pat. No. 4,677,677 to Eriksson and U.S. Pat. No. 4,677,676 to Eriksson disclose systems for analyzing and producing the noise cancellation signals that must be delivered to a cancellation point. U.S. Pat. No. 4,876,722 to Decker et al and U.S. Pat. No. 4,783,817 to Hamada et al. disclose particular component locations which relate to the performance of the cancellation, but does not otherwise discuss how such systems are to be constructed, particularly in a manner which would render them applicable to muffle engine noise in the environment of a motor vehicle.
Moreover, the previously known systems often employ extremely large transducers such as 12 or 15 inch loud speakers of conventional construction. Such components are not well adapted for packaging within the confines of the motor vehicle, and particularly, within the under carriage of the motor vehicle. Moreover, the low frequency content of the signals which must be cancelled is on the order of 25 hertz. Furthermore, the highest frequencies encountered on the order of 250 hertz. Conventional wisdom suggests that a large loudspeaker would be necessary to generate sound signals with sufficient amplitude in that frequency range. Such speakers are particularly impractical to mount beneath the motor vehicle. Furthermore, while many of the prior art references teach installation of the speakers within the ducts carrying the sound pressure signal, such a mounting is impractical in the environment of motor vehicle exhaust conduits. In addition, while the limited area for exhaust conduit routing might suggest that the size of a speaker to be used in an active noise cancellation muffler would be reduced in size and compensated for by additional speakers of small size, such a multiplication of parts would substantially increase the cost of producing the active muffler system while at the same time having an adverse impact upon reliability of such a system.
In addition, from a production and manufacturing standpoint, the transducer and its driving circuit represents substantial portion of the cost of the system. In particular, the sensing and processing apparatus can be miniaturized to a great degree, and thus may have minimal packaging and materials impact. On the other hand, the speaker may include a large magnet, and the driving circuit includes power transformers to generate large amplitude signals required to drive the transducer or loudspeaker emitting the cancellation pulses. Moreover, the larger components in the power circuit increase cost not only by the expense of the individual components in the circuit but also by adding to the temperature compensation components and costs to control the heat generated in the power system.
Moreover, typical transducers are usually designed for optimum operation at room temperature environmental conditions. In contrast, the motor vehicle exhaust system typically attains temperatures hundreds of degrees above normal environmental temperatures. Depending upon the material used in the construction of the transducer magnet, the operating temperatures of the motor vehicle have an adverse impact upon the flow of flux through the magnetic flow path. In particular, it is well recognized that the flow of magnetic flux in typical transducers will diminish as the magnet is subjected to higher and higher temperatures. As a result, at the typical high temperatures of the vehicle operating environment, a substantially greater amount of power must be provided by the power circuit in order to operate the transducer at a level which will effectively cancel the noise pressure pulses passing through the exhaust conduit. Thus the use of conventional components in such system would substantially increase the cost as well as the packaging size of the components which must be used in order to provide active noise cancellation mufflers in motor vehicles.